The development of new forms of active substances such as drug compounds and pesticides, as well as a desire to increase the efficacy of existing substances, has created a need to develop new and effective ways of delivering substances to their appropriate targets. It is likely that many potentially useful active substances have not been commercialised because of inadequate formulation. In many cases, the inability to formulate the active substance into a deliverable form could simply be due to solubility problems.
Although useful as vehicles for the delivery of active substances, most emulsions and liposomes are limited by the fact they are thermodynamically unstable and, generally, over time, will coalesce and may eventually separate into two distinct liquid phases (emulsions) or will degrade and release the fluid-filled core into the surrounding media (liposomes). This instability is exacerbated in veterinary and pharmacological applications since the vehicles are used under circumstances (e.g. increased salt (electrolyte) or variations in pH) which put a severe strain on the vehicle structure. The degradation of vehicles containing active substances is undesirable since considerable time and effort is spent in formulating the delivery system. In the veterinary, pharmaceutical and nutriceutical industries in particular, if vehicle stability is compromised, the bioavailability of the active substance may be affected.
Particle stabilised emulsions are known, however, the stability of the resulting capsules remains poor over a period of time. This means that it is difficult to transport the capsules over long distances and it is difficult to store the capsules for a delayed time of use. As the capsules degrade, the active substance (e.g. a drug compound or a pesticide) within the capsules can leach out, or may be released without control. Leaching or uncontrolled release of active substances can pose a serious problem in the delivery of certain drugs in the body, since one intent of the encapsulation process is to shield healthy cells from the drug's toxicity and prevent the drug from concentrating in vulnerable tissues (e.g. the kidneys and liver).
Existing preparations of particle stabilised vehicles (capsules) are usually dispersed in a liquid in order that the capsules can be delivered to the body as a liquid suspension. These liquid formulations usually have a low active substance content to liquid ratio and, in addition, during storage or transport, there is a risk of microbial growth in the liquid which can cause serious infections or spoilage.
A further problem is coalescence of the capsules to form capsules with an increased diameter. Larger capsules are less stable over time, and larger capsules cannot be delivered to some areas where the diameter of the capsule will not be permitted (e.g. capillaries in the body). Further to this, active substance release profiles are correlated with interfacial surface area. It is important, therefore, that capsule size remain constant in order that the release profile of the active substance is maintained.
Accordingly, it is an object of the present invention to provide a capsule for the delivery and/or dry storage of an active substance which has a relatively long shelf-life and is therefore easy to store or transport and may have a reduced risk of microbial growth during storage.